1. Field of the Invention
The present invention relates generally to an ultrasonic surgical assembly and, more particularly, relates to a method of maintaining constant movement of a cutting blade of an ultrasonic waveguide, in particular, in a cordless, hand-held, fully electrically powered and controlled, surgical ultrasonic cutting device.
2. Description of the Related Art
Ultrasonic instruments are effectively used in the treatment of many medical conditions, such as removal of tissue and cauterization of vessels. Cutting instruments that utilize ultrasonic waves generate vibrations with an ultrasonic transducer along a longitudinal axis of a cutting blade. By placing a resonant wave along the length of the blade, high-speed longitudinal mechanical movement is produced at the end of the blade. These instruments are advantageous because the mechanical vibrations transmitted to the end of the blade are very effective at cutting organic tissue and, simultaneously, coagulate the tissue using the heat energy produced by the ultrasonic frequencies. Such instruments are particularly well suited for use in minimally invasive procedures, such as endoscopic or laparoscopic procedures, where the blade is passed through a trocar to reach the surgical site.
For each kind of cutting blade (e.g., length, material, size), there are one or more (periodic) driving signals that produce a resonance along the length of the blade. Resonance results in optimal movement of the blade tip and, therefore, optimal performance during surgical procedures. However, producing an effective cutting-blade driving signal is not a trivial task. For instance, the frequency, current, and voltage applied to the cutting tool must all be controlled dynamically, as these parameters change with the varying load placed on the blade and with temperature differentials that result from use of the tool.
FIG. 1 shows a block schematic diagram of a prior-art circuit used for applying ultrasonic mechanical movements to an end effector. The circuit includes a power source 102, a control circuit 104, a drive circuit 106, a matching circuit 108, a transducer 110, and also includes a handpiece 112, and a waveguide 114 secured to the handpiece 112 (diagrammatically illustrated by a dashed line) and supported by a cannula 120. The waveguide 114 terminates to a blade 116 at a distal end. A clamping mechanism, referred to as an “end effector” 118, exposes and enables the blade portion 116 of the waveguide 114 to make contact with tissue and other substances. Commonly, the end effector 118 is a pivoting arm that acts to grasp or clamp onto tissue between the arm and the blade 116. However, in some devices, the end effector 118 is not present.
The drive circuit 104 produces a high-voltage self-oscillating signal. The high-voltage output of the drive circuit 104 is fed to the matching circuit 108, which contains signal-smoothing components that, in turn, produce a driving signal (wave) that is fed to the transducer 110. The oscillating input to the transducer 110 causes the mechanical portion of the transducer 110 to move back and forth at a magnitude and frequency that sets up a resonance along the waveguide 114. For optimal resonance and longevity of the resonating instrument and its components, the driving signal applied to the transducer 110 should be as smooth a sine wave as can practically be achieved. For this reason, the matching circuit 108, the transducer 110, and the waveguide 114 are selected to work in conjunction with one another and are all frequency sensitive with and to each other.
Because a relatively high-voltage (e.g., 100 V or more) is required to drive a typical piezoelectric transducer 110, the power source that is available and is used in all prior-art ultrasonic cutting devices is an electric mains (e.g., a wall outlet) of, typically, up to 15 A, 120 VAC. Therefore, all known ultrasonic cutting devices resemble that shown in FIGS. 1 and 2 and utilize a countertop box 202 with an electrical cord 204 to be plugged into the electrical mains 206 for supply of power. Resonance is maintained by a phase locked loop (PLL), which creates a closed loop between the output of the matching circuit 108 and the drive circuit 106. For this reason, in prior art devices, the countertop box 202 always has contained all of the drive and control electronics 104, 106 and the matching circuit(s) 108. A typical retail price for such boxes is in the tens of thousands of dollars.
A supply cord 208 delivers a sinusoidal waveform from the box 202 to the transducer 110 within the handpiece 112 and, thereby, to the waveguide 114. The prior art devices present a great disadvantage because the cord 208 has a length, size, and weight that restricts the mobility of the operator. The cord 208 creates a tether for the operator and presents an obstacle for the operator and those around him/her during any surgical procedure using the handpiece 112. In addition, the cord must be shielded and durable and is very expensive.
Another disadvantage exists in the prior art due to the frequency sensitivity of the matching circuit 108, the transducer 110, and the waveguide 114. By having a phase-locked-loop feedback circuit between the output of the matching circuit 108 and the drive circuit 104, the matching circuit 108 is required always to be located in the box 202, near the drive circuit 108, and separated from the transducer 110 by the length of the supply cord 208. This architecture introduces transmission losses and electrical parasitics, which are common products of ultrasonic-frequency transmissions.
In addition, prior-art devices attempt to maintain resonance at varying waveguide 114 load conditions by monitoring and maintaining a constant current applied to the transducer. However, the only predictable relationship between current applied to the transducer 110 and amplitude is at resonance. Therefore, with constant current, the amplitude of the wave along the waveguide 114 is not constant across all frequencies. When prior art devices are under load, therefore, operation of the waveguide 114 is not guaranteed to be at resonance and, because only the current is being monitored and held constant, the amount of movement on the waveguide 114 can vary greatly. For this reason, maintaining constant current is not an effective way of maintaining a constant movement of the waveguide 114.
Furthermore, in the prior art, handpieces 112 and transducers 110 are replaced after a finite number of uses, but the box 202, which is vastly more expensive than the handpiece 112, is not replaced. As such, introduction of new, replacement handpieces 112 and transducers 110 frequently causes a mismatch between the frequency-sensitive components (108, 110, and 112), thereby disadvantageously altering the frequency introduced to the waveguide 114. The only way to avoid such mismatches is for the prior-art circuits to restrict themselves to precise frequencies. This precision brings with it a significant increase in cost.
Some devices claim to be able to contain all necessary components for ultrasonic procedures within a single handle. These devices, however, do not currently appear in the marketplace and the written descriptions of each disclose virtually no details of how their circuitry is enabled. At least one such device is described as being completely sealed and all of the device's electronic components, such as the power supply and the transducer, are non-replaceable. This design is self-evident, because the tool, used in surgery, must be sterilizable. However, in some surgeries, a cutting tool reaches its maximum lifespan within very few surgeries or, in some cases, even before the surgery is finished. With a sealed device design, the entire device must be disposed, including its expensive internal components.
In addition, this device is described as using inductive charging. It was not designed or envisioned to use modern, long-lasting, high-power batteries, such as lithium-ion (Li) batteries. As is known in the art, Lithium batteries cannot be charged in a series configuration of multiple cells. This is because, as the voltage increases in a particular cell, it begins to accept charging energy faster than the other lower-voltage cells. Therefore, each cell must be monitored so that a charge to that cell can be controlled individually. When a Lithium battery is formed from a group of cells, a multitude of wires extending from the exterior of the device to the battery is needed. Sakurai cannot provide this necessary feature because, by design, the sealed autoclavable Sakurai device does not and cannot have a plurality of external exposed contacts to be coupled to a charging device. In fact, the inductive charging feature for the sealed device is entirely at odds with exposed contacts.
Therefore, a need exists to overcome the problems associated with the prior art, for example, those discussed above.